On this site tags are used particularly by users of PY Ads (see the brown bar above); the tags they use within their advert are used to identify the same tags in the many articles. When someone visits an article it will look for corresponding tags in the PY Ads. When it finds any one of the tags it will display the PY advert in the right-hand column of that article.

For the general site visitor it can be a way of finding information on a subject of interest, more specific than a general search that does a search through the article’s text and will pick up part-words eg search for abs and it will also pick up abstract, absorb etc.